Some questions about cory cats?

Your cory would very much appreciate you turning the tank to sand, it is more like their natural environment, and it does not damage their barbels. I have heard a lot of different things about sand, and the ones I hear most consistently are make sure the sand has smooth, rounded grains, playsand usually works well. Make sure to wash wash wash the sand, and then when you think you've washed it enough, wash it some more. The cons of sand are that if your filter intake is too close to the sand it will ruin your filter, it won't work with undergravel filters. Pros are that it is great for planted tanks, and is usually more aesthetically pleasing. You should up your number of cories to at least 3, make sure it is 3 of the peppered, they won't shoal with the other variations. I would try to rehome that tetra, 3 of the peppered cories will pretty much max your tanks stocking, and besides the tetra is in all likelihood a schooling fish. I would look at these two sites to come up with your stocking plan. . .